The present invention relates to a thin-film transistor (TFT) using polycrystalline silicon thin-film as an active layer serving as a source area or a drain area, and a liquid crystal display unit (LCD) using such TFT.
There is at present a demand for a liquid crystal display unit capable of displaying at a higher speed, and as one of the means for satisfying such a requirement, it has been tried to change an active layer such as a gate area, a source area, or a drain area of the switching thin-film transistor that controls the liquid crystal layer from the amorphous silicon thin film into the polycrystalline silicon. This is the result obtained by paying attention to the fact that the mobility of the carrier in polycrystalline silicone is higher than that in amorphous silicon in principle.
In addition to formation of a polycrystalline silicon thin film having a high carrier mobility on an insulating substrate, it has been tried, not to externally attach a semiconductor chip having a single-crystal silicon active layer to the driving circuit of the liquid crystal display section as in the conventional art, but to simultaneously form a thin-film transistor having an active layer comprising a polycrystalline silicon thin film in the frame of the pixel section on the same substrate from the beginning.
We will now briefly describe with by referring to the drawings the process of forming a thin-film transistor by polycrystallizing this amorphous silicon, after forming an amorphous silicon thin film on a transparent insulating substrate such as a glass substrate, and the manufacturing method of a liquid crystal display unit using a thin-film transistor having an active layer comprising this polycrystalline silicon thin film, since they are related with the intent of the present invention although they may fall under the known conventional art.
FIG. 1 illustrates the state of changes in the cross section of a conventional thin film transistor using a polycrystalline silicon thin film as an active layer according to the progress of manufacture. Actually, a large number of the thin-film semiconductor for the pixels and the driving circuits thereof are formed and arranged in many rows and stages, i.e. the upper, lower, right, and left on the substrate in accordance with the layout of the liquid crystal display section. However, since this is a well known fact, and moreover, it is troublesome to show this process in detail, only one thin-film semiconductor is shown in this FIG. 1.
In this FIG. 1, 1 represents an insulated substrate having transparency such as a glass, 2 represents a buffer layer for preventing an alkali metal or the like contained in the insulated substrate 1 from diffusing into an active layer comprising a silicon thin film and exerting an adverse Effect, 3 represents an amorphous silicon thin film; and 4 represents a polycrystalline silicon thin film. 5 represents a gate insulating film (layer) comprising, for example, SiO2 and Si3N4 and 6 represents a gate electrode. 7 represents a channel area and 8 represents a source area. 9 represents a drain area, and 10 represents a contact hole. 11 represents a source electrode, and 12 represents drain electrode.
A manufacturing method of a thin-film transistor using a polycrystalline silicon thin film as an active layer of thin-film transistor will now be described sequentially by referring to FIG. 1.
(a) The amorphous silicon thin film 3 shall be formed by deposition on the insulated substrate via the buffer layer 2;
(b) Polycrystallizing treatment of silicon shall be performed by applying a heat treatment to the amorphous silicon thin film 3 to. More particularly describing, the polycrystalline silicon thin film is obtained by irradiating an excimer laser (beam) onto the amorphous silicon thin film 3, instantaneously melting the amorphous silicon, causing crystallization in accordance with cooling, and finally applying so-called laser annealing (or laser anneal technique). Then, unnecessary portions of the polycrystalline silicon thin film 4 on the substrate shall be removed, and the gate insulating film 5 and the gate electrode 6 shall sequentially be formed on the substrate.
In this state, impurities determining a type of conduction of the polycrystalline silicon thin film 4 such as phosphorus (P) or boron (B) are introduced from the upper section of the substrate into the polycrystalline silicon thin film 4 to form the source area and the drain area of the thin-film transistor, by using the gate electrode 6 as a mask, or simultaneously using a resist together if necessary, so as to prevent impurities from entering the channel area 7. This introduction is usually accomplished by injecting P or B ions accelerated with a high voltage. A case with P ions are illustrated in FIG. 1.
(c) The source area 8 and the drain area 9 are formed by activating the impurities through a heat treatment carried out by irradiating an excimer laser onto the polycrystalline silicon thin film 4 again.
(d) The source electrode 11 and the drain electrode 12 are formed by forming the contact hole 10, and by burying a metal inside it.
Next, the thin-film transistor shown in FIG. 1 is of so-called top-gate type in which a gate insulating layer is arranged on the substrate side of the gate electrode. As the applicable thin-film transistors for a liquid crystal unit include, apart from the top gate type, there is a type known as the bottom-gate type in which the gate insulating layer is arranged on the opposite side of the substrate against the gate electrode.
The bottom-gate type is advantageous in that it is capable to almost perfectely prevent impurities from diffusing from the undercoat such as the glass substrate to the channel area by means of the gate metal electrode. In this structure, however, because impurities forming the source area and the drain area cannot be diffused from the relatively thick substrate side, diffusion will be made from the silicon layer side after forming the silicon layer. As the result, it becomes difficult, or even impossible to perform self-alignment for forming the channel area, thereby causing deterioration of the transistor characteristics, such as a increased gate capacity.
On the other hand, a favorable feature of the top-gate type is that impurities forming the source area and the drain area is injected, from the gate electrode side after forming the silicon layer, by using the gate electrode as a mask, and thereby self-alignment will be permitted for forming the channel area. In this structure, however, since there is no gate metal under the channel area, diffusion of the impurities from the undercoat such as a glass substrate into the channel area during the subsequent heat treatment cannot completely be prevented, or is at least difficult to completely prevent such diffusion. If the thickness of the undercoat insulating film layer on the substrate is increased to avoid this defect, various problems such as cambering of the substrate will occur.
A conventional bottom -ate type thin-film transistor and a manufacturing method of it will now be described in detail by referring to the drawings.
FIG. 2 illustrates formation of the cross-section according to the progress of the manufacturing process of the conventional bottom-gate type thin-film transistor. In this FIG. 2, the numeral (latter we omit xe2x80x9cthe numeralxe2x80x9d) 1 represents a transparent insulated substrate Comprising a glass or the like. 5b represents a gate insulating layer comprising SiO2 or the like, and 6b represents a gate electrode. 7b represents a channel area in the silicon semiconductor layer, and 8b represents a source area in the silicon semiconductor layer. 9b represents a drain area in the silicon semiconductor layer, and 30 represents photo-resist. 5c represents an interlayer insulating layer, 11b represents a source electrode, and 12b represents a drain electrode.
The manufacturing method thereof will be described as based on the FIG. 2.
(a) The gate electrode 6 shall be formed on the transparent insulated substrate 1, and then the gate insulating layer 5b shall be formed covering the upper section of it;
(b) The silicon semiconductor layer 4 (a necessary area only) shall selectively be formed on the gate insulating layer 5b. In this step, when using polycrystalline silicon, which is attracting the general attention at present, as a silicon semiconductor layer, this amorphous silicon layer shall be polycrystallized by annealing using an excimer laser through a laser anneal, with a laser anneal technique, for example, after an amorphous silicon layer is formed.
Subsequently, a photo-resist 30 is formed only on the upper section of the silicon semiconductor layer at a position which is to constitute an upper section of the gate electrode 6b, and then B and other impurities determining a type of conduction of silicon is injected from the upper section of the substrate onto the silicon semiconductor layer by using this photo-resist 30 as a mask. As the result, the channel area 7b in which impurities constituting the thin-film transistor are not existent, the source area 8b and the drain area 9b into which impurities are injected will be formed.
(c) By forming an interlayer insulating layer 5c on the entire surface of the substrate after removing the photo-resist, by opening the contact hole 10 at a position corresponding to the source area 8b and the drain area 9b in the interlayer insulating layer 5c, and by incorporating the metals such as Ti and Mo by sputtering or the like into this contact hole to form the source electrode 11s and the drain electrode 12b, manufacturing of the thin-film transistor will be finished.
However, the thin-film transistor manufactured through the steps shown in the preceding FIG. 1 cannot be made into a single silicon crystal at the present level of art. In order to realize the liquid crystal display on a large screen of over 12 inches to 20 inches, or a further larger screen of 30 inches, uniformity of the thin-film transistor elements and the functions are still insufficient, and in is turn, uniformity of the display on a liquid crystal display unit and the display functions are still also insufficient.
Next, the bottom-gate type thin-film transistor shown in FIG. 2 has the following problems.
First, because a silicon semiconductor layer to serve as a source area and a drain area is present on the upper side of the gate electrode, it is necessary to form a photo-resist as a mask at a position corresponding to the gate electrode which is readily formed, when injecting impurities into the silicon semiconductor layer, a positional alignment at this point will also be required. However, for the extra-compact and fine thin-film transistor for the liquid crystal display of higher-grade fineness in a conceivable future, it will be difficult to conduct such a positional alignment. To describe just for information, a thin-film transistor at present is going to have the gate width of 10 xcexcm, the length of about 6 xcexcm, and the transistor length of about 20 xcexcm, and further downsizing is expected to be realized in the future.
Under these circumstances, the top-gate type thin-film transistors are becoming the mainstream.
Secondly, the technique of injection of the impurity ions accelerated with a high voltage is used as a means for introducing the impurities into the currently polycrystallized silicon semiconductor layer, irrespective of the top-gate type or the bottom-gate type. However, this technique will more or less cause a damage to the crystal lattices of the silicon semiconductor layer. Therefore, a heat treatment is applied for recovering such a damage. The temperature for such a heat treatment is, however, limited to up to about 600xc2x0 C. at the maximum because of the heat resistance of the glass of the substrate. In its turn, it is difficult to completely recover the damages.
Further, because of the very thin undercoat layer and semiconductor layer, alkali metals diffuse from the glass substrate into the semiconductor particularly during this heat treatment. Consequently, this leads to deterioration of the performance of the semiconductor.
Under these circumstances, there has been a demand for the development of a thin-film transistor excellent in high pixel density as well as response, and further having a sufficient performance in terms of the response and quality uniformity for the liquid crystal display units having a large display screen in the conceivable future.
Whether the top gate type or the bottom gate type, there has been a demand for the development of a technique adopting an inexpensive glass substrate, and less susceptible to damages to the silicon layer or the like when injecting the impurities. The demand has been particularly strong for the bottom-gate type.
Also, in an extra-fine bottom-gate type thin-film transistor, there has been a demand for the development of a technique permitting the injection of impurities appropriately coping with the gate electrode.
In addition, there has been a demand for the achievement of an inexpensive liquid crystal display unit having a very high pixel density and a satisfactory response with a large display area by using such a thin-film transistor.
Further, by paying attention to the decrease in the melting point during annealing and the considerable electric field movement, a new semiconductor thin film was recently developed by adding carbon and/or germanium located adjacent to silicon in the periodic table (5% carbon at the maximum, or up to 30% germanium a the maximum) in place of pure silicon. However, because similar problems are encountered in this case as well, there has been a demand for solving such problems.
The present invention was developed for the purpose of solving the problems as described above.
For this purpose, the first group of aspects of the invention provides a thin-film transistor having a polycrystalline silicon thin film serving as an active area formed on an insulating substrate, in which crystal grains of the polycrystalline silicon thin film have anisotropically been grown at an array substrate end within a plane, and in a direction in parallel with, or at right angles to, the gate length direction of the thin-film transistor in parallel therewith, or at right angles thereto, and the longitudinal direction thereof is at a particular angle to the gate longitudinal direction of the thin-film transistor.
When the anisotropic growth direction of the crystal grains is in the gate length direction, some of barriers are eliminated upon movement of the carrier, thus improving the electric field effect and mobility.
At this point, from 0.5 to 2 grains of the polycrystalline silicon thin film are contained per micron (1 xcexcm) of the gate length. As the result, it is possible to achieve an electric field effect as typically represented by mobility of at least 300 cm2/Vs for all the thin-film transistors.
Also, uniform transistor properties are available by manufacturing the thin-film transistor so that the longitudinal direction of the grains is substantially in parallel with the length direction of the gate of the thin-film transistor.
At this point, 5 to 20 grains are contained per micron of the gate length. This is desirable in terms of the uniformity and a high mobility.
The longitudinal growth direction of grains forms an angle of 45xc2x0 to the gate length direction of the thin-film transistor. This brings about a thin-film semiconductor showing a good balance between high-speed movement and uniform characteristics.
It is desirable that from 1 to 10 grains are contained per micron of the gate length.
From the point of view of operating efficiency, it is desirable to anneal the upper (lower) and left (right) driving circuit sections and pixel sections by a long and slender laser beam in a run of scanning, and this is adopted in the invention.
Further, the substrate or the laser beam, particularly the substrate, is moved in a direction substantially at right angles to the longitudinal direction, in which the intensity of the laser beam in the shorter side direction is stronger at least at the center portion.
Next, in a liquid crystal display unit, provision of the driving circuit of the pixel section on the same substrate leads to a lower cost as a whole. Particularly, use of the driving circuit on the data drive side permits achievement of a high-performance liquid crystal display unit excellent in driving characteristics.
Further, when a shift register is provided in the driving circuit section, a more compact liquid crystal display unit is available. Provision of a buffer circuit as required results in a further more excellent display unit. Use of the thin-film transistor as a liquid crystal switch for the pixel section permits achievement of a liquid crystal display unit giving a good contrast. Such a liquid crystal display unit is proposed in the present invention.
The second group of aspects of the invention provides a bottom-gate type thin-film transistor formed on the substrate having an undercoat insulating layer formed on the substrate, which contains impurities determining a type of the conduction of silicon, in which the impurities contained in the silicon layer in contact with the undercoat layer upon laser annealing of amorphous silicon are diffused to form the source area and the drain area.
Further, the presence of the undercoat insulating layer prevents impurities contained in the transparent insulating substrate from diffusing into the thin-film semiconductor. As such an undercoat layer, a BSG (boron silicate glass) layer or a PSG (phosphorus silicate glass) layer is used.
In the manufacturing method of the thin-film transistor using a polycrystalline silicon semiconductor for the polycrystallization of amorphous silicon through irradiation of an excimer laser beam, it is instantaneously exposed to a very high temperature. A high-melting-point metal is therefore used as a gate electrode material.
Further, the high-melting-point metals mainly comprising Cr, Mo, Ti and the like are used to facilitate formation of a gate side wall insulating layer through oxidation of the gate electrode. Moreover, since Ti or Cr is passivated in oxidation (oxidizable), the thickness of the oxide film is spontaneously controlled. This is extremely favorable in the case of an extra-fine element.